This invention relates to hydraulic shock absorbers of the type used for land vehicles and is particularly directed to an improved shock absorber for motorcycles.
In recent years the social requirement of motorcycle safety has become more and more prominent. Improvements in vehicle stability have heretofore been made, but sometimes at the expense of riding comfort. Accordingly, it is an important object of this invention to provide an improved hydraulic shock absorber which results in both an improvement in vehicle stability and also in riding comfort.
In analyzing vehicle stability and comfort as affected by the shock absorbers of a vehicle suspension system, resort to representative damping characteristics curves is believed helpful. FIG. 1 of the drawings provides a comparison of a number of such representative curves. Conventional damping systems for motorcycles generally exhibit a damping capacity characteristics curve labeled "B" in FIG. 1. A system employing dampers exhibiting the characteristics of curve "B" transmit a trembling feeling to the rider's hands when running on a rough surface. The low speed resistance force is higher than desirable and the relative discontinuity at the point designated "I" tends to transmit strong impacts in the continuous movement of the system. The type of rough surfaces effecting such vibration includes worn paved roads, roads full of repair work and roads with rain grooves. As such conditions can be encountered for extended periods of time, such an unfavorable response curve can result in increased operator fatigue.
In recognition of such deficiencies in the more conventional shock absorber mechanisms, efforts have been made to create a more favorable damping capacity characteristics curve. To this end, French Pat. No. 1,342,210 issued to Societe Anonyme Andre Citroen discloses a device which employs multiple mechanisms to arrive at a total effect having a damping capacity characteristics curve indicated by curve "C" in FIG. 1. The addition of a plate valve in the Citroen device does, in a somewhat discontinuous manner, eliminate the sharp rise in resistance at low piston velocities. The sharp discontinuities shown at points "II" and "III" in curve "C" of FIG. 1 result in strong impacts to the continuous movement of the damper rod. The strong impacts induce large vibrations of the handlebars or seat of a motorcycle and trembling feelings on rough road surfaces similar to what is experienced by the abrupt slope change of curve "B" at point "II" in FIG. 1.
Using known techniques curve "A" of FIG. 1 may also be achieved. However, this curve lacks damping capacity in the range of low piston speeds and high damping force at midrange. The lack of damping at low range piston speeds reduces handling responsiveness while the large damping at the mid-range piston speeds detracts from operator comfort. Consequently, the more conventional systems heretofore employed for motorcycle shock absorbers have compromised one advantageous characteristic for another.